She attended a morning church service, joining in the carol singing led by a cassock-wearing choir, and then watched a nativity play performed by children from the congregation.

But Miss Zhang's Protestant church is an illegal one, and its 1,000-strong members have grown used to worshipping in a variety of office buildings across Beijing in an effort to avoid the scrutiny of the authorities.

A 25-year-old graduate and junior manager in an engineering company, Miss Zhang has been a Christian for four years. She says many people, including her parents who are local government officials and members of the communist party, think she's "crazy" and question both her faith and the wisdom of being a Christian in a communist country.

"They say, 'There's no God in this world,'" she said. "They haven't stopped me being a Christian and I wouldn't let them, but it's a source of tension between us. I pray for them."

But unlikely as it sounds, Miss Zhang is part of a huge and growing number of like-minded Chinese who were celebrating the story of Christ's birth on December 25. Up to 100 million are now practising Christians - possibly a higher proportion of the country's 1.3 billion people than in Britain - and they outnumber the 76 million members of the Chinese Communist Party (CCP).

While Christmas in Beijing and other major Chinese cities is regarded by many people as just an excuse to go shopping, it is also the time whenChina's Christians are most visible. In Beijing, the official government-run churches were as packed as the unofficial ones. Outside the south cathedral, the capital's largest and oldest Catholic church, the queue of people hoping to attend midnight mass stretched for more than 400 yards. Ushers in red Father Christmas hats greeted worshippers with "Shengdan Kuaile!", the Chinese for "Happy Christmas", as 1,500 people crammed into the pews for the mass which was led by the Bishop of Beijing, Li Shan. Hundreds more watched the service on two giant television screens outside the cathedral, singing along to carols including Silent Night. Yet most Chinese Christians, like Miss Zhang, prefer to worship in illegal "house churches". It is a brave choice in a country where the authorities remain deeply suspicious of organised religion, and especially Christianity. "Buddhism and Islam aren't regarded as Western religions, so the CCP is more opposed to Christianity," said Cao Zhi, a Christian who works for the World and China Institute, an independent Beijing-based group that provides legal aid to Chinese Christians. "The party don't really know anything about Christianity, but they regard it as a Western ideology and so they are opposed to it." The Chinese constitution officially guarantees religious freedom but in an effort to monitor their religious activities, Christians, whether Roman Catholic or Protestant, are required to attend state-controlled churches - a stricture ignored by those who meet instead in flats, anonymous office blocks, university dormitories and even the private rooms of restaurants. The authorities are increasingly clamping down on their activities, fearing they could be used to organise opposition to the communist party's rule. Last year, a house church network with 50,000 members in Linfen in northern Shanxi Province was shut down and its leaders imprisoned. In the last three months, house churches in three other provinces have been closed, while in October 200 members of house churches were prevented from travelling to Switzerland to attend the 3rd World Congress on Evangelisation, one of the biggest worldwide gatherings of Protestants. "There's always conflict," said Li Baiguang, an activist lawyer who was one of three Chinese Christians invited to the White House in 2006 by former President George W Bush. "Sometimes, the police just take away the furniture from the house churches to try and stop people gathering in them. Other times, they'll arrest the priests or pastors, saying it's an illegal gathering." Mr Li won a national endowment for democracy award in 2008 for his work defending Christians who have been arrested for practising their religion. He is convinced the crackdown on Christians is intensifying. "In the last four years I've been involved in many cases involving house churches. Ninety per cent of the people I defend get convicted," he said. "The courts will nearly always support the authorities. Priests and house church leaders are getting arrested more frequently. The government feels threatened by them and is afraid of their power." Yet the number of Christians continues to grow. Jin Tianming, the pastor of the house church which Miss Zhang attends, said: "When we started in 1993 there were just 10 of us. Now, we have more than 1,000 members and 10 full-time staff. Every Sunday there are around 30 newcomers." With room for only 300 worshippers in the room near the Beijing Film Studios where the church meets, Mr Jin has to hold three services on Sundays to accommodate everyone who wants to come. He believes China's huge transformation over the last three decades has fuelled Christianity's growth. "Things have changed so much and so quickly that people need faith," he said. "Before they believed in communism, now they need something else. And while the changes have given people the opportunity to make money, they have also introduced more pressure into peoples lives." When The Sunday Telegraph attended the church, worshippers mostly under the age of 40 occupied every seat and stood lining the walls, many clutching Bibles and prayer books in both Chinese and English. Prayers ended with loud shouts in English of "Hallelujah" and "Amen". Some stood with their hands above their heads, palms held upwards, praying for the holy spirit to enter them. Mr Jin preached on the persecution early Christians faced. It was an apt sermon, given the troubles his church has faced. "We often get harassment from the local government," he said. "At first, when we met in residential apartments, the police would find out and we'd have to move. Since 2005, we've met in office buildings but we keep being forced to move because the local authorities put pressure on the landlords. Last year, we were evicted in November and had to pray outside in a park in the snow." Christians began to go underground soon after the communist takeover in 1949, even though Christianity in China dates back to the seventh century. Beijing severed diplomatic ties with the Vatican in 1951 and ordered Catholics to join a state-run Catholic patriotic association, which does not recognise the primacy of the pope. An official Chinese Protestant church was also set up. Those Catholics who remained loyal to Rome were forced to worship in secret, and many of the far more numerous Protestants followed suit. New Christians are particularly drawn to the fervent, evangelical Christianity of the Protestant house churches, which has spread from nearby South Korea - one of the most Christian countries in Asia. Korean missionaries have been active in China since the early 1980s while posing as ordinary businessmen and women. According to the government's own figures around 28 million people are members of the official churches, meaning that up to 72 million worship in secret. "I feel the pastors in the official churches are controlled by the government and I have more freedom to worship in a house church," said Mr Li. "Most Christians in China feel the same." Those who attend the house churches are almost all loyal to the government, but resent its interference in their religious activities. "People who go to house churches believe politics shouldn't have a part in religion. Last year it was the 60th anniversary of the founding of the new China, and the official churches had to hold services of celebration," said Mr Cao. "That would never happen in a house church." Equally worrying for the CCP is the spread of Christianity's appeal from the poverty-stricken countryside, where it first took root, to the young, university-educated residents of China's fast-swelling cities - the very people from whom the party traditionally recruits. Chen Yan, 23, a trainee manager at a software company who attends Mr Jin's church, said this was partly because of the stress of modern life, with pressure to get into a good university and then a job. "But I also think young people are thinking more about the meaning of life and not just about getting ahead," she said. "When I started coming to church, I felt less troubled. I felt God was giving me the confidence to face life." Most Chinese Christians are introduced to religion by friends or family. Public advocacy of Christianity remains almost impossible. "There's no way you could stand on the street giving out leaflets telling people to read the Bible," said Mr Li. "That would be very dangerous." But few Chinese Christians seem put off by the authorities' distrust of their religion. "It's no problem if the government doesn't like Christians or house churches," said Miss Zhang. "God is in charge of us, not the government." Some names have been changed.